This invention relates to a method for producing oxide superconductors, and more particularly to a method for producing oxide superconductors, the superconductivity transition temperature (hereafter referred to as Tc) of which is higher than the boiling point of liquid nitrogen (absolute temperature 77 K.).
As an oxide superconductor having a superconductivity transition temperature exceeding the boiling point of liquid nitrogen (e.g. Tc=80 K.), RBa.sub.2 Cu.sub.4 O.sub.8 (R=Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, or Tm) of a three-layer perovskite type crystal structure with double CuO chains is known [Phys. Rev. B., 38, 7347 (1989)]. The crystal structure of this superconductor has been determined as shown in FIG. 1. It is also known that Tc can be raised up to 90 K. by substituting Ca for a part of Y [Nature, 341, 41 (1989)]. Recently, the present inventor has found that an oxide superconductor substituted with Sr for a part of Ba in the above-mentioned oxide superconductor can have a lesser content of toxic Ba and yet keeps its superconductivity unchanged.
The oxide superconductor (Y,Ca)Ba.sub.2 Cu.sub.4 O.sub.8, in which Ca has been substituted for a part of Y mentioned above, is conventionally produced by the following process. Material powders of the constituent elements are blended to have a predetermined element ratio, and the mixture is calcined and is subjected to a hot static pressing process for about ten hours in a mixed atmosphere of oxygen gas (20%) and argon gas (80%) at 2000 atm. Almost the same process was applied to the oxide superconductor containing Sr substituted for a part of Ba.
The hot static pressing process for a long time, necessary for the above-mentioned production method, is disadvantageous for use in mass production, and it makes oxide superconductors of this system expensive.